i) Field
The present relates to the removal and recovery of an acid from a mixture of soluble non-ionic organic compounds and acid. A non-ionic organic compound/acid stream can be generated after biomass hydrolysis with acids or the production of organic acids (e.g. lactic acid, succinic acid, acetic acid) from sugars using fermentation or other means. In particular, the process described herein may be employed to remove the acid from an acid/sugar stream obtained after pulp hydrolysis with sulphuric acid or other acids to produce nanocrystalline cellulose (NCC). This approach enables the recycle of the acid to the hydrolysis step and avoids the use of chemicals to neutralize the stream and the generation of wastes. The de-acidified sugars can be fermented or converted to valuable fuels or chemicals.
ii) Description of the Prior Art
The annual biomass growth on the earth is estimated to be 118 billion tons. The production of fuels and biochemicals from biomass feedstocks is attractive and sustainable. During such processes, the lignocellulosic material is hydrolyzed to break down the polysaccharide components (e.g. cellulose and hemicellulose) into monomeric and oligomeric sugars before converting them to valuable products. This hydrolysis step involves generally the use of a significant amount of mineral acid. Sulfuric acid is generally the acid of choice due to its availability and cost. Different acid concentrations ranging from less than 0.5% to 80% are used to release the sugars. The acid plays the role of a catalyst and is not consumed during the hydrolysis step. After the hydrolysis step, the acid is generally neutralized with lime before sugar fermentation. As a result, significant amounts of calcium sulfate can be generated which need to be dealt with. This practice increases the operating costs associated with the production of sugars and, by extension, sugar-derived biofuels and chemicals. To make the hydrolysis step more economically attractive, the spent acid should be separated and recycled to the process. The recovery and recycle of acid will reduce the cost of the sugar conversion and the cost of waste disposal.
The separation of electrolytes from nonelectrolytes in different applications has been investigated in the prior art. Several approaches such as chromatographic techniques, nanofiltration, reverse osmosis, and crystallization have been suggested and investigated. Chromatographic techniques include ion exchange, ion exclusion and ion retardation. In ion exchange systems, ions (cations or anions) are exchanged between the solute and a resin. Nonelectrolytes in solution have no interaction with the resin and pass straight through the resin bed. Thus, they can be separated from electrolytes using this technique. Several applications of this approach have been devised. Regeneration of the resin is generally performed using chemicals.
In ion exclusion, there is no exchange of ions between the solute and the resin. This technique is used to separate ionic from nonionic (or weakly ionic) species. This technology employs a microporous resin which can sorb water and nonionic solutes. Electrolytes such as sulfuric acid are prevented from entering the porous resin structure due to ion repulsion. Therefore, an electrolyte will pass faster than a nonelectrolyte through a column packed with such a resin. Thus, in ion exclusion it is expected that the acid is eluted first from the resin bed while the sugar exits second because it penetrates deeper in the porous resin. Ion exclusion has been used mainly in analytical and pharmaceutical applications since it is limited to low flows and low concentrations of species.
U.S. Pat. No. 5,403,604 dealt with sugar separation from juices using a set of membrane units including ultrafiltration, nanofiltration (NF) and reverse osmosis. The sugars were retained by the NF membrane while acids such as citric acid passed through. The total acid concentration in the feed stream was about 0.79 wt % while the total sugar varied from 4.3 to 14.3%.
U.S. Pat. No. 7,077,953 dealt with acid recovery from a hydrolysate solution obtained after exposing wood chips to an acidic solution. In this case, the sugars and the acid were contaminated with several other compounds such as lignin, metals, and suspended solids. The inventor used a chromatographic unit to retain and separate most of the sugars from the hydrolysis process. Water was employed to elute the sugars from the chromatographic unit. The eluted sugars were sent to a processing unit such as a fermentation/distillation unit. The chromatographic unit yielded a dilute sugar stream which upon fermentation yielded a diluted product that will require more energy to concentrate. The acid-rich stream from the chromatographic system was processed using a nanofiltration unit to remove the remaining sugars. The inventor also suggested having a second nanofiltration unit ahead of the chromatographic unit to concentrate the sugars. This approach involves the use of several steps which may not be economically viable.
U.S. Pat. No. 5,580,389 discussed the separation of acid from sugars from strong acid hydrolysis of biomass. The method involves several steps such as removal of silica, de-crystallization, hydrolysis, and sugar/acid separation. The latter separation was performed using a cross linked polystyrene cation exchange resin to retain the sugars. The resin was cross-linked with 6 to 8% divinylbenzene and treated with sulfuric acid to produce a strong acid resin. The resin was then washed with water to release the sugars. The sugar solution could thus be fermented to produce value added-products.
U.S. Pat. No. 7,338,561 describes a process for purifying an aqueous solution containing one or several sugars contaminated with multivalent cations, monovalent metal cations, monovalent anions and multivalent inorganic anions and/or organic acid anions. The process employs several separation units including: a strong anionic resin, a strong cationic resin, a nanofiltration device, a crystallization unit, a reverse osmosis unit, and up to two chromatographic columns. This approach was applied to a permeate from an ultrafiltration unit treating whey. Chemicals are needed to regenerate the columns filled with the anionic and cationic resins. The use of all of these units to perform the desired separation is complicated and does not seem to be economically attractive. Also, it is indicative of a low separation efficiency.
Hatch, M. J. and Dillon, J. A., Industrial and Engineering Chemistry Process Design and Development 2(4), 253, October 1963 used an acid retardation resin to separate acids from salts. A similar acid retardation resin has been employed, for example, to purify the waste generator acid (U.S. Pat. No. 5,792,441) produced at kraft pulp mills.
U.S. Pat. No. 5,968,362 describes a method for separating acid and sugars from a biomass acid hydrolysis step. The process employed an anionic exchange resin or an ion-exclusion chromatographic material in a simulated moving bed (e.g. from Advanced Separation Technologies) to retain the acid from the hydrolysate. The sugars produced were contaminated with acid and metals. The author proposed a treatment with lime to neutralize the solution and precipitate the metals.
U.S. Pat. No. 5,628,907 describes the separation of acid-sugar mixtures using ion exclusion chromatography. The separation of glucose from sulfuric acid at different feed concentrations and different modes of operations was reported. Several resins with different degree of divinylbenzene (DVB) cross-linking were employed.
There is still a need for an acid/sugar separation method that is simple and efficient. The method should be more economically viable compared to the other approaches mentioned above which employ several separation steps thereby having high capital and operating costs. Such a method will preferably minimise the dilution of the sugar and acid product streams.